I mainly use Twitter for town-level information (as a glorified RSS), and the local administration barely masters Twitter, let alone more complex tools. But if there is a simple way to copy their messages towards the Fediverse, that could help bootstrap it.
There's also a decent, chance your posts are already readable on Mastodon through https://bird.makeup/. I wouldn't use that as an official source, but it's probably good to know about. It should also be noted that some servers block dedicated Twitter forwarding services like these because of the massive moderation task blindly forwarding the Twitter cesspool can cause.
I can't tell you what tool or program suits your needs most. There are systems ranging from fully custom ActivityPub servers to simple set-and-forget IFTT automations (https://ifttt.com/explore/how-to-crosspost-mastodon-twitter) that can solve the problem of "post the same stuff on two platforms". Sadly, the most easy to use tools that I know of are useless without access to the Twitter API and I doubt you'll be willing to spend the ridiculous amounts of money Twitter demands for that for a few replicated tweets.
As far as simple guides, everyone has their own idea of what "simple" means. In its most basic form, you type or paste text into an input field and hit the send button, just like on Twitter. You give people your username to follow (@user@server.com),they type that into their dashboard and they hit the follow button. If you want to reply to people, you click the reply button and type a reply. Retweeting and liking works the same way as on Twitter, with similar icons. Usernames are a bit longer because they include a domain name, but honestly who even cares about usernames on social media.
If you've ever emailed someone outside your organisation, using Mastodon shouldn't be harder than using Twitter. The hard part is synchronising the two, because the only people who have put effort into that so far either used the Twitter API or are comfortable with running shell scripts.
perhaps try WordPress out with the ActivityPub plugin?